Perils of Plutonium

Scientists fret over storage of spent nuclear fuel in pools

Is the highly radioactive spent fuel being stored in ponds at reactor sites secure against terrorist attack from a small plane carrying explosives or from a gang armed with short-range missiles? A recent report by a panel of the National Academy of Sciences, in Washington, D.C., raises questions about the situation in the United States and, by implication, in other countries as well [see photo, "Inviting Target?"].

The report, which was released in April and gave rise to some worried commentary in the general press, said that "attacks [against spent-fuel ponds] by knowledgeable terrorists with access to appropriate technical means are possible." Such an attack could partly or completely drain a pool, so that the fuel would ignite, resulting in the "release of large quantities of radioactive materials to the environment."

The panel stated that it had not been able to get all the information it needed to adequately assess the situation at all U.S. plants, and it recommended that an organization independent of both the nuclear industry and nuclear regulators be given the job of taking a closer look. For prompt protection of vulnerable pools, the panel suggested two relatively simple measures to reduce the immediate risk of fires and radiation release: facilities could reposition fuel assemblies for better heat distribution, and they could install backup sprinkler systems to cool assemblies if they are completely drained.

Photo: Lisa Powell/Ap Photo/The Gazette

Inviting Target?

Spent fuel rods cooling in pools--like the fuel assemblies in this pond at the Duane Arnold Energy Center in Palo, Iowa--could tempt terrorists to attack with missiles or planes.

The U.S. Nuclear Regulatory Commission (NRC) responded to the report's release with a statement conceding the advisability of further analysis. But the statement from its chairman accompanied a rebuttal that the NRC had prepared before the NAS report was complete, an unusual procedure. The gist of the NRC's rebuttal was that "today, spent fuel is better protected than ever"--a claim that met with some skepticism. "It is disturbing that the [nuclear regulatory] commission... denied the academy the information needed to assess the effectiveness of security improvements instituted since 9/11," said The New York Times in an editorial. The NRC denies it withheld information from the panel and says misunderstandings arose in connection with what information could be given to the public.

Currently, 104 Commercial Nuclear Reactors in the United States generate electricity from uranium fuel that is slightly enriched in fissile U-235. The resulting spent-fuel rods, which no longer have enough fissile material to sustain reactions efficiently, contain a mix of uranium and plutonium as well as many highly radioactive by-products that generate intense heat and radiation. Reactors in the United States have produced more than 50 000 tons of spent fuel since 1968, and the U.S. Department of Energy predicts that the total will exceed 63 000 tons by 2008.

The spent-fuel assemblies must cool for at least five years in pools--generally pits 11 meters deep on average, filled with about 380 000 liters of circulating water--before they can be safely handled. After five years, they can be removed and stored in dry casks made of steel and cement. Now, though there are dry-storage facilities at more than two dozen power plants, most plants are choosing to fill their pools to capacity before switching to dry storage.

According to the NRC, roughly a fifth of U.S. spent fuel is in dry casks. About 42 000 tons of it is in pools, and of that, about 34 500 tons has been in them more than five years.

The academy's report says that dry storage has "inherent security advantages" over pools, because it is a passive system that relies only on air cooling and because it divides the fuel into small quantities that are stored in robust containers. In principle, it would be desirable to move spent fuel into dry storage as soon as possible, the report implies, and once fuel is stored that way, there may be no urgent need to secure it further.

A second report, issued in May by the public affairs panel of the American Physical Society (APS), concludes that dry-cask storage is good enough to take care of U.S. spent fuel for decades to come, even if construction of the permanent storage facility planned for Yucca Mountain in Nevada is further delayed. "Interim storage of spent fuel in dry casks--either at current reactor sites or at a few regional facilities or at a single national facility--is safe and affordable for a period of at least 50 years," says the APS report.

By the same token, the APS panel concludes that there is no urgent need for the United States to reprocess spent fuels, to extract plutonium for reuse in fresh fuel. Two years ago, the Department of Energy, in a controversial move, shifted the focus of its Advanced Fuel Cycle Initiative research program. It now considers reprocessing a partial solution to the disposal problem, because it reduced the volume of highly radioactive nuclear material needing to be stored. But critics argue that reprocessing doesn't make storage easier, because when the radioactive isotopes of elements like strontium and cesium are separated, they have to be dealt with in special ways. Fuel that hasn't been reprocessed is also more proliferation-resistant, because its high-level radiation discourages theft.

"Our argument is you don't need reprocessing plants to have nuclear power, though you do need enrichment plants [to produce the fuel for reactors]," says APS panel member John Ahearne, director of the ethics program of Sigma Xi, a science honors society based in Research Triangle Park, N.C.

In its report, the APS panel concluded that any decision about reprocessing, as well as any decision about construction of a second permanent fuel repository to supplement Yucca Mountain, can be "comfortably deferred, and should be deferred, for at least a decade." Deserving more attention now are pool storage perils.